Process for preparing molded structure from polyphenylene sulfide resin and filler

ABSTRACT

THE MELTING TEMPERATURE OF THE RESIN; THE MOLDING PRESSURE OF ABOUT 3000 P.S.I. IS APPLIED TO THE MOLD BEFORE THE RESIN HAS TIME TO ENCAPSULATE THE FILLER; AFTER COOLING THE MOLD WHILE MAINTAINING THE PRESSURE, THE MOLDED STRUCTURE IS REMOVED FROM THE MOLD.   A NOVEL MOLDED STRUCTURE OF POLYPHENYLENE SULFIDE RESIN AND A FILLER IS PRODUCED ACCORDING TO A PROCESS WHEREIN THE RESIN/FILLER MIXTURE IS PRECURED BY HEATING THE MIXTURE TO A TEMPERATURE BELOW THE MELTING TEMPERATURE OF THE RESIN PRIOR TO MOLDING; THE MOLD IS PREHEATED TO A TEMPERATURE OF ABOUT 600 TO 700*F, AND THE CHARGE OF RESIN/FILLER MIXTURE IS PREHEATED TO A TEMPERATURE BELOW

Feb. 13, 1973 J. c. TRoccloLA ErAL 3,716,609

. PROCESS FOR PREPARING MOLDED STRUCTURE FROM POLYPHENYLENE SULFIDE RESIN AND FILLER Filed Oct. 5. 1970 2 Sheets-Sheet 1 Feb. 13, 1973 1 c. TRoccloLA ETAL 3,716,609

PROCESS FOR PREPARING MOLDED STRUCTURE FROM POLYPHENYLENE SULFIDE RESIN AND FILLER Filed oct. 5, 1970 2 sheets-snaai 2 Q LL M QM P5 QP QS m 5 y@ -No N m RR w -u United States Patent O Conn.

Filed Oct. 5, 1970, Ser. No. 77,928 l Int. Cl. B221? 3/14; B29c 25/00; C08g 25 /00 U.S. Cl. 264-111 y 5 Claims ABSTRACT` F THE DISCLOSURE A novel molded structure of polyphenylene sulfide resin and a ller is produced according to a process wherein the resin/lillervmixture lis precured by heating the mixture to a temperature below the melting temperature of the resin prior to molding; the mold is preheated to a temperature of about 600 to 700 F. and the charge of resin/filler mixture is preheated to a temperature below ther melting temperature of the resin; the molding pressure of about 3000-p.s.i. is applied to the mold before the resin has time to encapsulate the iiller; after cooling the mold while maintaining the pressure, the molded structure is removed from the mold.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a molded structure, and particularly to molded fuel cell coolant plates in which polyphenylene sulfide resin is utilized as a binder.

Description of the prior art Polyphenylene sullide resin has been blended with ller materials such as graphite, carbon, silver, copper, iron and magnesium. However, in known methods of combining the polyphenylene sulfide resin with the filler material, a precuring step is performed after mixing the resin and ller at a temperature above the melting temperature of the resin. When so precured, the resin tends to encapsulate the filler; the encapsulation has a detrimental etect on the electrical and thermal characteristics of the composition. That is, the electrical resistivity is relatively high and the thermal conductivity is relatively low, both properties being undesirable when the composition is used, for example, as a fuel cell coolant plate or the like where it is desired that the plate has a high rate of thermal conductivity and low electrical resistivity.

It is also known that polyphenylene sulfide resin/filler mixtures can be molded into desirable shapes by conventional molding techniques. However, we have discovered a molding process by which the electrical conductivity and thermal conductivity of the molded structure are signiiicantly increased over molded structures made according to known processes.

SUMMARY OF THE INVENTION The object of the present invention is to provide a process for molding a structure of polyphenylene sulfide resin and a filler and the article produced thereby by which the molded structure acquires superior thermal and electrical conductivity compared to structures made by known processes.

The above and other objects and advantages are obtained by a process in which polyphenylene sulfide resin is mixed with a filler (the resin powder comprising to 40 percent of the mixture by weight), either before or after precuring the resin, the essential feature of the process being that the resin is heated to a temperature below its melting point during precure so that the resin does not encapsulate the iiller. Further in this regard, during the -molding process, it is desirable to preheat the mold to a temperature of 600 to 700 F. After the charge of resin-tiller has been loaded into the mold, it is necessary to apply the molding pressure (of about 3000 p.s.i.) before the resin has time to encapsulate the ller and case. Encapsulation of the filler prevents good filler to filler contact and results in poor electrical and thermal conductivity of the molded structure.

In further accord with the present invention it has been found that preheating the resin-filler mixture to a temperature below the melting temperature of the polyphenylene sullide resin before loading the mixture into the preheated mold produces a molded structure, the electrical resistivity of which is only about 60 percent as great as the electrical resistivity of a mixture preheated to a temperature above the melting point of the resin.

Our novel process for making a molded polyphenylene sulfide resin-filler structure with greatly increased thermal t and electrical conductivity can, for example, be ac- A preferred composition has been found to be 85 weight percent graphite powder and l5 Weight percent resinY powder. Precure the resin (either before or after it is blended with the iller) by heating it to a temperature below its melting temperature, which is about 550 F.; baking at about 500 F. for about l2 hours in a circulating air oven has produced very satisfactory results. Place preheated (to about 500 F.) resin-filler mixture into a preheated (600-700 F.) mold and apply a load of about 3000 p.s.i. and maintain the mold at a temperature above 550 F. for about live minutes; it should be noted that the molding pressure should be applied before the resinfiller mixture heats up to a temperature above the melting point of the resin in the preheated mold so that the resin does not encapsulate the graphite.

The mold and contents are then cooled to about 400 IF., and it is preferable to maintainfull pressure during cooling. The molded structure may then be removed from the mold.

The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a preferred embodiment thereof.

BRIEF DESCRIPTION O-F THE DRAWINGS DESCRIPTION O-F THE PREFERRED EMBODIMENT A molded structure, which may be, for example, a cooling plate for use in fuel cells, may be made of polyphenylene sulfide resin and a ller such as nickel, graphite,

. magnesium, aluminum, copper, tantalum, iron, titanium,

silver or other suitable materials.

Polyphenylene sulfide resin is thermosetting in nature, but is not a true thermoset. 'Ihis characteristic enables the resin-filler composition to be compression or injection molded, for example, like a thermoplastic, while still maintaining the high thermal stability of a thermoset. The resin is a free owing powder with a particle size of 25 to 170 microns. The particle size distribution of the resin is shown in FIG. l. The resin has a melting point of about 550 F.; it tends to harden to the temperature at which itis cured, butV does not reach an infusible state.

The particle size of the filler is important in order to get improved thermal and *electrical conductivity. We have `iound that `the particle size ofthe ller which gives As seen in the above table, the electrical resistivity'is about 50 percent greater if the powder is preheated to above the melting temperature of the resin (Test No. l). This demonstrates that the resin has encapsulated the optimum results is to 150 microns. The particle size 5 graphite particles. distribution of ller is shown in FIG. 2. The physical andl structural properties demonstrated -by The polyphenylene sulfide resin must be precured belo an 85 weight percentl graphite and 15Y weight percent polyits melting temperature prior to molding in order to phenylene sulfide structure molded in accordance with achieve a homogenous structure and a structure which has our invention are shown in ,Table II.

Y TABLE n Direction parallel-.to Bulk a pressing Property: A

Density 1.95 gm./cm.3 r Electrical resistivity (70 F.) 11x10-3 ohmcm. ThermalV conductivity (200 F.) 13.5 B.t.u./hr. ft. F. .Compressive creep (1,500 p.s.i. at 300 F.) 1 Compressive strength (70 F.) v -14,600 p.s.i. Compressive strength (300 F.) '1

Corrosion resistance:

06 w/o HzPOq (100 hrs.

H10 autoclave (500 hrs.

KOH', 12.5 w/o Kroos 1,000 hrs. at 200 FJ- at 300 III 015% wt. 'gai ni f 11,400 p.s.i."

the greatly improved thermal and -electrical conductivity of our molded structure. The resin powder and the filler powder can be blended before or after precuring of the resin since the resin is precured below its melting vtemperature. In other Words, when the resin is precured below its melting temperature, there is no possibility that it can encapsulate the iiller particles, even if the resin is precured after it is blended with the ller particles. The encapsulation of the iiller would result in poor thermal and electrical conductivity as will be shown hereinafter.

After the resin and filler have been blended and the resin precured, (not necessarily in that order), the actual molding process might v take place as follows to obtain a high density, highly conductive structure: A mold is preheated to about 600 to 700 F. The precured resin-ller mixture is preheated to a temperature below the melting point of the resin (500 F., for example, produces .excellent results). The reason for preheating the precured resin-ller mixture is t0 decrease heatup timeof the mixture in the mold and to decrease the heat loss from the mold. Once again it is essential that the mixture not be preheated to the melt temperature ofthe resin because the filler particles would be encapsulated: by the resin, which hasa very higlrwetting ability and would inhibit good filler particle to particle Contact, which would result in poor electrical and thermal conductivity. The mold is then loaded with the desired charge of preheated resin-filler mixture, and a load of about.3,000 p.s.i. is quickly applied to the mold; the load Ashould be Table `Il demonstratesthat the proper-riesci he graphite-polyphenylene vsulfide resinV structures are emrnently suitable for both acid and base el'ectrolyte'e`- cell applications.v L To obtain a measure ofthe molding cha cteristics f the polyphenylene sulde resin and iiller mixture, nioldiig trials were conducted to compare the 110W properties of 85 weight percent graphite/ 15 weight percent polyphenylene sulfide resin composition and an A85 weight percent graphite/ 15 Weight percent Resinoxcomposition. AResinox is a phenolic resin which has been usedheretofore in molded structures. Cold pressed two inch diameter discs of the compositions were placed in asix inch b y six inch mold andv thencompressed. A relative measure of flow was obtained by comparing the'area's of thediscs after pressing under a 3000 p.s.i. load. The graphite/polyphenylene suliide resin composition flowed to a 70 percent greater area than the graphite/Resinox composition.

There hasvthus been described aprefcrred embodiment of a process vfor molding a polyphenylene,sullegesin iiller composition, in ,accordancewith ouninven lIt should be unde stoodby those skilled in the art tha varie o us changes and ornissionsin the form and detailthereof may be made thereinlwithout. departing fron1 ,th e spirit applied before the resin has time to heat up to its melt temperature so that encapsulation of the ller particles does not occur. While maintaining full pressure the mold is maintained above 550 F. (the melt temperature of the resin) for about live minutes until proper curing of the resin occurs. The mold is then cooled to about 500 to 400 F. while maintaining full pressure. After so cool- A.

TABLE I Temperature before ressin F. v p g Electrical Moldingl Test; Resinfiller pressure, Deuslty, reslstlvi y, number Mold mixture p.s. gin/cm. ohm-cm.

Whatwe claim as novel and desire tosecure by Letters Patentof the United States is: y y, 1- ALYPIOClSS- f Or mldng'lauel cell platehaving proved thermal conductivityE end electrical'conductivity comprising: Y Y

mixing polyphenyle e sulfide resin powderhaving -a particle size of 25 to 170 microns .withat :least :one i ller powder selected from tlieigroup consisting Yof nickel, graphite', carbon, magnesium, aluminum, copper, tantalum, iron, titanium-and silver havinga particle size of 1 0 to 150Y microns, Vsaid resin'powder comprising 10 to 10 perce-nt ofthejmixture byweight;V precuring ysaidl polyphenylene sulfide Vresin-'byheating it to a temperature below its melting'`temp"'erature before or after :friiiidng'withts'aid fillerpowder and holding at temperature for a sufficient period ottime to provide a. substantially uniformtemperature dis-1 .-tribution throughout said resin;

preheating a mold to a temperature of, 600 to 7.00"l F.; loading said moldfwith a charge of resin/ ller mixture which has been preheated to a temperature below' ller, while maintaining said mold at a temperature above the melting temperature of the resin;

cooling said mold while maintaining said load; and

removing the molded structure thus produced from said mold. 2. A process for molding a structure as defined in claim 1, wherein the particle size of the ller powder is in the range of 10 to 150 microns.

3. A process for molding a structure as defined in claim 1, wherein the filler mixed with the resin is graphite. 4. A process for molding a fuel cell plate having improved thermal conductivity and electrical conductivity comprising:

mixing polyphenylene sulfide resin powder having a particle size of 25 to 170 microns with at least one ller powder selected from the group consisting of nickel, graphite, carbon, magnesium, aluminum, copper, tantalum, iron, titanium and silver having a particle size of 10 to 150 microns, the resin powder comprising about 15 percent of the mixture by weight;

precuring said polyphenylene sulfide resin by heating it to about 500 F. before or after mixing with said filler powder and holding at temperature for a sufficient period of time to provide a substantially uniform temperature distribution throughout the resin;

preheating a mold to a temperature of about 600 to preheating a charge of the resin/filler mixture to a temperature of about 500 F.;

loading said preheated charge of resin/filler mixture into said preheated mold;

applying a load of about 3000 p.s.i. to said mold before said resin-filler mixture heats up to a temperature above the melting point of the resin in the preheated mold, so that the resin does not encapsulate said filler, while maintaining said mold at a temperature above 550 F.;

cooling said mold to about 400 to 500 F. or lower while maintaining full pressure; and

removing the molded structure from the mold.

S. A process for molding a structure as defined in claim 4, wherein the filler mixed with the resin is graphite.

References Cited UNlTED STATES PATENTS 3,166,615 1/1965 Farrell 264-23 3,539,672 11/ 1970 Valyi 264-1 11 3,592,783 7/1971 Edmonds 260-37 R 3,487,454 12/1969 Oates et al 260-79.1 3,354,129 1l/1967 Edmonds et al 260-79 3,562,199 2/ 1971 Hill et al. 26o-79.1

ROBERT F. WHITE, Primary Examiner 25 J. R. THURLOW, Assistant `Examiner U.S. C1. XR. 

